This invention relates to a pipe with spiral internal ribs which run with rotational symmetry with the longitudinal axis of symmetry of the pipe.
A known pipe of this type which is described according to German Utility Model No. 74 22 107 has several multiple-thread screw-like internal ribs on its inside, having a small width b and a small radial height e. The width b should be in the range of 0.02 to 0.15 inch and the height e should be in the range between 0.0125 and 0.075 inch; i.e., under the assumption that 1 inch=25.4 mm, the greatest extent in both dimensions should be a width b of 3.8 mm, a height e of 1.9 mm and an inside diameter of approximately 20.3 mm. It follows from this that turbulence which promotes heat transfer will develop in the vicinity of the inside wall in such a pipe through which a fluid is flowing because of the ratio of the inside diameter of the pipe to the relatively short inside ribs which are designed to be like nubs in cross section, but there is no secondary flow across the main direction of flow, and thus ultimately the heat transfer effect remains limited to the main flow conditions and the turbulence caused by irregularities in the wall.
The inventor of German Patent No. 196 09 641 C2 which pertains to a different generic field has recognized this disadvantage of the heat transfer surface of the internal ribs being too low and to this end proposed a pipe for cooling concrete floors with air, said pipe being provided with much longer, straight internal ribs which extend radially from the inside wall of the pipe in the direction of the longitudinal axis of symmetry. However, this pipe has the disadvantage that core flow, i.e., flow through the free central space near the longitudinal axis of symmetry, is subject to a considerable pressure drop, and effective heat transfer between this core flow and the inside wall of the pipe is left up to chance because there is no flow across the main direction of flow which would increase the heat transfer. Because of wall friction, the flow within each of two adjacent rib flanks and the partial chamber formed by the rib flanks and the inside wall of the pipe has a lower velocity than the core flow. Furthermore, the mass exchange between the core flow and the flow in the individual chambers is left up to chance. Since these ribs lower the heat transfer coefficient due to the reduced rate of flow in the chambers, their positive effect is based exclusively on the fact that they increase the heat transfer area. The same thing also applies to the pipe according to FIG. 2 of German Patent 27 03 341 C2, which is of a different generic type.
In addition, European Patent No. 0 582 835 A1 describes a heat transfer device which is composed of several pipes of a different generic type, where the outside walls are graduated, and additional pipes with different dimensions and inside ribs in various configurations are arranged concentrically in the interior to serve as an oil cooler. These heat transfer pipes have the disadvantage of a considerable pressure drop, in addition to the fact that they are expensive to manufacture, because there is little or no cross-flow, which could increase the heat transfer, or such cross-flow can develop only randomly and remains limited to the internal pipe.
In addition to the publications mentioned above, there is also an extensive state of the art pertaining to pipes having internal ribs as described, for example, in Unexamined German Patent No. 24 02 942, German Patent No. 33 34 964 A1 and Unexamined German Patent No. 26 15 168, all of which concern internal ribs having the disadvantages explained above. Since there is no spiral twist to these ribs, they also do not conform to the generic type of the pipes described in the present invention.
The object of the present invention is to create a heat transfer pipe of the generic type defined in the preamble, which is characterized by a much better heat transfer performance in comparison with the pipes having internal ribs known in the past, and to this end it not only causes an increase in the internal heat transfer area but also guarantees an effective cross-current between the inside wall surface of the pipe and the core flow in the vicinity of the longitudinal axis of symmetry to increase the heat transfer.
This object is achieved by the following features according to this invention in conjunction with the generic term described in the preamble:
a) The free ends of the internal ribs are the same distance a from the longitudinal axis of symmetry of the pipe, said distance a being in a ratio in the range of 1:12 to 1:3 relative to the inside diameter d of the pipe
b) All the inside ribs run in a spiral twist in the same direction and with the same length of the spiral relative to the longitudinal axis of symmetry.
Due to these features, a pipe is created for the first time which not only has a large heat transfer area on its inside due to the small distance a between {fraction (1/12)} and ⅓ of the inside diameter of the pipe, but also a cross-flow with a relatively low pressure drop develops, thus ensuring a considerable increase in the heat transfer effect between the core flow and the wall of the pipe, said cross-flow developing due to the spiral twist of the inside ribs in each spiral twisted interspace between two adjacent rib flanks and the pipe wall on the one hand and the core flow on the other hand flowing through the free space near the longitudinal axis of symmetry. This active principle does not have any precursor in the entire state of the art, whether because no marked cross-flow can develop due to the short nubby ribs according to the most proximate state of the art as described in German Utility Model No. 74 22 107, but instead only an increased turbulence can develop in the wall area, or whether it is because of the fact that the longer ribs according to the state of the art do not do not have any spiral twist.
In the design of the cross-sectional shape of the inside ribs, this invention permits several embodiments.
According to a first embodiment, the cross-sectional shape of each rib forms an acute equilateral triangle with straight legs, with the tip of the triangle developing into the two legs in a rounded form with a radius, where two adjacent internal ribs form an interspace having a trapezoidal cross section. Although this cross-sectional shape is basically known from German Patent No. 33 34 964 A1, the ribs there do not have a spiral twist, so they cannot be regarded as known in combination with the spiral twist features of claim 1.
According to a second embodiment, the cross-sectional shape of each internal rib of the pipe is in the form of a tooth with toothed wheels having flanks with an outward convex curvature and with the tips of the teeth being rounded, with two adjacent ribs forming an interspace having a U-shaped cross section with the side faces having a concave curvature. This rib shape is especially suitable for high-viscosity fluids such as oils.
According to a third embodiment, the cross-sectional shape of each internal rib forms an equilateral acute triangle with legs that form a concave inward curvature and a semicircular shape at the tip, where two adjacent internal ribs surround in a U shape an interspace having a trapezoidal cross section, the trapezoid legs having a convex outward curvature. This rib shape is preferably used with low-viscosity fluid flow such as gases.
All these different embodiments of the internal ribs lead to different flow patterns across the core flow in the area of the longitudinal axis of symmetry. The number of ribs, the pitch of the twist, the rib thickness and their shape are designed as a function of the type of fluid and its rate of flow as well as the pressure drop without thereby going beyond the scope of this invention.
According to an especially advantageous refinement of this invention, these pipes are mass produced with their internal ribs made of extruded aluminum or copper or extruded plastic. Both aluminum and copper are characterized by a high thermal conductivity.
To ensure a uniform core flow and cross-flow, the cross-sectional shape of the pipe with its internal ribs and the interspaces is the same over the entire length of the twist in each cross-sectional level.
The wall thickness of the pipe is determined as a function of the system pressure and is advantageously in a range between 0.4 mm and 3 mm, with each pipe having at least four internal ribs.
To obtain the greatest possible heat transfer effect with a relatively low pressure drop, the distance a of the free ends of the internal ribs from the longitudinal axis of symmetry of the pipe is greater in the case of high-viscosity fluids such as oils and is lower in the case of lower-viscosity fluids such as water and gases. This causes an increase in the cross section of the core flow in the area of the free cross section in the vicinity of the longitudinal axis of symmetry in the case of high-viscosity fluids in comparison with low-viscosity fluids.
According to this invention, the free interior in the vicinity of the longitudinal axis of symmetry in each pipe may by no means be closed. This space must communicate with the channels between the ribs. For this reason, in an advantageous refinement, the free ends of the internal ribs are always a distance a away from the longitudinal axis of symmetry, even in the case of low-viscosity fluids, so that a core flow channel is maintained between its free ends in each cross section of the pipe. For this reason, according to feature a of the main claim, this distance a is no less than {fraction (1/12)} the inside diameter of the pipe.